System and method for selected pump LEDs with multiple phosphors

ABSTRACT

An LED pump light with multiple phosphors is described. LEDs emitting radiation at violet and/or ultraviolet wavelengths are used to pump phosphor materials that emit other colors. The LEDs operating in different wavelength ranges are arranged to reduce light re-absorption and improve light output efficiency.

This application is a continuation of U.S. application Ser. No. 15/077,387 filed Mar. 22, 2016, now allowed, which is a continuation of U.S. application Ser. No. 13/211,145 filed on Aug. 16, 2011, now U.S. Pat. No. 9,293,667 which claims the benefit under 35 U.S.C. §119(e) of U.S. Application No. 61/502,212 filed on Jun. 28, 2011, all of which are hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

This invention is directed to lighting systems, and in particular, to light emitting diodes (LED) pump light with multiple phosphors.

Solid state lighting is known. Solid state lighting relies upon semiconductor materials to produce light, e.g. by light emitting diodes. Red LEDs are known and use Aluminum Indium Gallium Phosphide or AlInGaP semiconductor materials, among others. Most recently, Shuji Nakamura pioneered the use of InGaN materials to produce LEDs emitting blue light. The blue LEDs led to other innovations such as solid state white lighting and the blue laser diode.

High intensity UV, blue, and green LEDs based upon the InGaN material system have been proposed and demonstrated. Efficiencies have typically been highest in the UV-violet, but drop off as the emission wavelength is increased to blue or green. Unfortunately, achieving high intensity, high-efficiency InGaN-based green LEDs has been problematic. Additionally, InGaN-based LEDs have been costly and difficult to produce on a wide-scale in an efficient manner. Although successful, solid state lighting techniques must be improved for full exploitation of their potential.

SUMMARY OF THE INVENTION

This invention provides pump LED light of selected wavelengths with multiple phosphors. In various embodiments, LEDs emitting radiation at violet and/or ultraviolet wavelengths are used to pump phosphor materials that emit light of a different frequency. The pump LEDs are characterized by having a peak emission wavelength of about 405 to 430 nm under normal operation. They are employed in conjunction with at least a blue phosphor with strong absorption at wavelengths beyond about 405 nm. In certain embodiments, LEDs operating in different wavelength ranges are arranged in a combination to reduce radiation re-absorption and improve light-output efficiency.

This invention provides an optical device which includes a mounting member and at least one light emitting diode overlying a portion of the mounting member. The LED includes a gallium and nitrogen containing substrate having a surface region and a gallium and nitrogen containing buffer layer overlying the surface region. An active region emits electromagnetic radiation with peak wavelengths in a range from about 405 nm to about 430 nm. The LED includes electrical contacts to supply the junction region electrical current. The device additionally includes a mixture of three phosphor materials within a binder material. The mixture of phosphor materials is disposed within a vicinity of the LED interacts with the electromagnetic radiation from the LED to convert the electromagnetic radiation to a wavelength range between about 440 to 650 nanometers. In another embodiment the device includes blue phosphor material within a vicinity of the LED device with strong absorption at wavelengths longer than about 405 nm. The active region is configured to emit electromagnetic radiation whose peak is in the range of about 405 to 430 nanometers while maintaining an internal quantum efficiency of about 70% and greater at a current density of at least 100 A/cm² in an operating temperature range from about 100° C. to about 150° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the efficiency characteristics of LEDs emitting at various wavelengths;

FIGS. 2 and 3 illustrate absorption properties of blue phosphors and the corresponding white LED performance depending on the emission wavelength of the LED;

FIG. 4 illustrates an LED with peak emission in the range of about 405-430 nm pumping blue, red, and green phosphors.

FIG. 5 illustrates carrier spreading in multi-quantum well LEDs emitting at 450 nm (left) and 420 nm (right);

FIG. 6 illustrates pump LEDs configured as an array;

FIG. 7 illustrates pump LEDs configured as an array, with a pixelated phosphor composition;

FIG. 8 illustrates pump LEDs configured as an array with two LED emission wavelengths; and

FIG. 9 is a simplified diagram illustrating emission spectrum of LED a two-phosphor violet pumped white LED according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed to lighting systems and to the provision of LED pump light with multiple phosphors. LEDs emitting radiation at violet and/or ultraviolet wavelengths are used to pump phosphor materials that emit different colors. Preferably the pump LEDs have a peak emission wavelength of about 405 to 430 nm under normal operating conditions.

As mentioned above, conventional LED lights sources are often inadequate. For example, one of the most-common approaches to generating high-CRI white LED light consists of LED devices emitting in the 440-470 nm range (often referred to as pump LEDs), which excite two phosphors: a yellow/green phosphor and a red phosphor. This approach is convenient because some yellow/green phosphors, such as Ce:YAG, exhibit high quantum efficiency.

Unfortunately, this approach is also constraining. YAG-based phosphors can only be efficiently excited in a narrow spectral range around 460 nm, limiting the wavelength range of the pump LEDs that can be employed. While high internal quantum efficiency (IQE) LED devices can be created at such wavelengths at low current density, their IQE falls rapidly at high current density. This is due to two effects: (a) the presence of large piezoelectric fields which reduce the carrier overlap and therefore increase the carrier lifetime, shifting the IQE curve to lower current density (ref); and (b) the challenges associated with thick active regions emitting around 445 nm (due to both strain-related growth limitations with thick InGaN layers and the difficulty with carrier spreading in a multi-quantum-well system).

FIG. 1 illustrates the efficiency behavior of LEDs emitting at various wavelengths. Short-wavelength LEDs (415-430 nm) maintain efficiency at much higher carrier densities than long-wavelength LEDs (445 nm and above). FIG. 1 is taken from “Influence of polarization fields on carrier lifetime and recombination rates in InGaN-based light-emitting diodes”, A. David et al, Appl. Phys. Lett. 97, 033501 (2010), which describes the increase of the polarization fields in longer-wavelength LEDs. The article “Carrier distribution in (0001)InGaN/GaN multiple quantum well light-emitting diodes”, A. David et al, Appl. Phys. Lett. 92, 053502 (2008), discusses the difficulty in spreading carriers between the quantum wells in 450 nm-pump LEDs.

The blue light from the pump LED contributes to the white spectrum. Therefore, the amount of blue light that is transmitted by the phosphors needs to be well-controlled to achieve a given CCT. Variations in the wavelength of the pump LED need to be accounted for in the phosphor composition/loading. Accounting for the differences in wavelength can be a challenging task when manufacturing white LEDs.

State-of-the-art results for 440-nm pump LEDs can be found in the article “White light emitting diodes with super-high luminous efficacy”, Y. Narukawa et al, J. Phys. D 43, 354002 (2010). At room temperature and a current density of about 100 A/cm², an external quantum efficiency of 65% is reported. Assuming an extraction efficiency of about 90% and a performance drop of about 10% between room temperature and a 100 C junction temperature, this corresponds to an IQE of about 65% at 100A/cm² and 100 C.

Another conventional approach consists in using a pump LED whose emission peak is in the 395-405 nm range to pump a system of three or four phosphors. This is advantageous because 400 nm pump LEDs typically retain higher performance at a higher current density than 445 nm pump LEDs, presumably due to the lower piezoelectric fields and to the thick active regions.

Employing an LED emitter whose final spectra is minimally affected (in color or brightness) by the presence or absence of the emitter wavelength in the final spectra allows for very stable performance over drive currents and temperatures. Proper selection of color-stable phosphor materials for the devices range of operation (e.g. a 405 nm emitter's spectral weight) is only 1.5% that of a 450 nm emitter. A 420 nm emitter's spectral weight is still only 10% of that of a 450 nm emitter. Stability in color and flux of a finished phosphor-converted LED is increased dramatically over a traditional blue pump device where as much as 20% of the final spectrum is comprised of the underlying emitter at 450 nm.

The elimination of the need to target a certain amount of emitter light leakage in the final spectra also offers improved color yield in a manufacturing environment. The manufacturing phosphor deposition process for an about 405 nm to 430 nm pump device can accept more process variance without sacrificing large-volume color repeatability. In turn, this allows a manufacturing process to run with higher throughput without loss in repeatability.

Use of three or more component colors (chip emission and at least two phosphors) offers a larger tunable color gamut for a phosphor-converted LED device than does a two-component color system. A large range of tunable colors and color rendering indices become available. A two-color component white LED will only have one possible cross section with the Planckian curve (one point to achieve a balanced white spectrum) whereas a three or more color system allows for infinite tunability along the Planckian curve.

This approach, however, suffers from various limitations:

-   -   1. The Stokes loss between the pump wavelengths and the         phosphors wavelengths is larger, so more energy is lost in the         phosphor down-conversion process. The comparatively larger band         gap of a 400 nm pump LED causes a larger operation voltage. The         reduced carrier confinement in the active region of a 400 nm         pump LED makes it easier for carriers to escape, and therefore         decreases the high-temperature performance. Most materials have         a significantly larger absorption at 400 nm than at 445 nm (this         is the case for high-reflectivity metals such as Al and Ag which         are commonly used in LEDs, of silicones, of some substrates such         as GaN or SiC, and of Au wirebonds.) which decreases the         light-extraction efficiency.     -   2. There has not been focused development on phosphors for use         with 380-430 nm excitation light. This places the performance         levels of the available phosphor materials behind the         state-of-the-art phosphor performance enjoyed by LED         manufacturers employing 450 nm pump LEDs for use with materials         such as Y₃Al₅O₁₂:Ce³⁺ (YAG-yellow) and CaAlSiN:Eu²⁺ (red) which         have had both time and pressure applied to their improvements.     -   3. Due to this offset in state-of-the-art phosphor material         performance, not all of the available phosphors can be used in         high performance LED devices. Prime examples are blue phosphors         which are not suitable for use with all chip-emission         wavelengths. The absorption characteristics of two blue-emitting         phosphors are shown in FIG. 2. Vertical lines indicate the         position of a 405 nm and 420 nm emitter relative to these two         phosphor absorption curves. Though the two materials absorption         strengths are similar at 405 nm, they are clearly not the same         at 420 nm. This reduction in absorption strength of the first         phosphor materially affects the device performance with         longer-wavelength emitters. This change in performance is shown         in FIG. 3.

This invention provides white LED light sources with high performance. Among other things, the invention provides a new approach to high-CRI white LEDs. For example, the white LED light source comprises pump LED(s) whose peak emission is in the range of about 405 nm to 430 nm and a system of three or more phosphors (such as blue, green, and red). A substantially white spectrum is produced by the phosphor emission.

One advantage of the invention is that a pump LED in the range of about 405 nm to 430 nm can display a very high IQE at high carrier density (similar to a 400 nm pump LED) due to the moderate strain and piezoelectric fields. The carrier confinement in the active region, on the other hand, is significantly improved so that the high-temperature performance is not compromised. The lower band gap compared to a 400 nm LED also enables a lower forward voltage. Therefore, the range of about 405 nm to 430 nm is optimal from the standpoint of pump LED performance. High IQE performance for such LEDs (at a current density of 100 A/cm² and a junction temperature 100 C) can be better than 70% and even exceed 90%. This is to be compared to about 65% for state-of-the art LEDs emitting at 440 nm as described in the prior art.

In addition, optical absorption in most materials is significantly reduced between 400 nm and about 405 nm to 430 nm, yielding overall larger light-extraction efficiency. Also, as explained above, the use of three or more phosphors to generate white light is advantageous in terms of color control and process stability. Blue phosphors are available with strong absorption in the range of about405 nm to 430 nm, and with high quantum efficiency. Some examples of blue-emitting phosphors withstrong absorption in this wavelength range are BaMgAl₁₀O17:Eu²⁺, Sr₁₀(PO₄)₆Cl₂:E, LaAl(Si_(6-z)Al_(z))N_(10-z)O_(z):Ce³⁺, a-Sialon :Ce³⁺, (Y, La)—Si—O—N:Ce³⁺, Gd_(1-x)Sr_(2+x)AlO_(5-x)F_(x):Ce³⁺. The Stokes loss is also mitigated in comparison to a 400 nm pump LED.

FIG. 4 illustrates an embodiment of the invention, where an LED whose peak emission is in the range of about 405 nm to 430 nm pumps blue, red, and green phosphors. As shown in FIG. 4, a pump LED source is provided on a substrate or a submount. For example, the pump LED source emits radiation at a wavelength of about 405 nm to 430 nm. The pump LED source is disposed in a mix of phosphor materials, which absorbs the radiation emitted by the LED source. The phosphor materials are excited by the pump LED and emit blue, green, and red light. In a preferred embodiment, the mix of phosphors is specifically adapted to emit white light, by the combination of emissions from the phosphors. The mix of phosphor materials is disposed in an encapsulant which is substantially transparent to both pump-LED-source and phosphor-emitted light.

Depending on the application, the encapsulant may include various types of materials. In a preferred embodiment, the encapsulant is specifically configured to improve light-extraction efficiency. For example, the encapsulant material can comprise polymeric species. In a preferred embodiment, the pump LED source emits radiation in the wavelength range from about 405 nm to 430 nm and pumps three phosphors (e.g., a blue, a green, and a red phosphor) that are mixed together, and the phosphor mix converts a substantial fraction of the pump LED source light to longer-wavelength light. Of course, the phosphor mix may contain additional phosphors, e.g. an amber phosphor can be added to increase CRI.

In various embodiments, the wavelength emitted by the LED changes due to changes in temperature. For example, a pump LED emits radiation at a wavelength of about 398 nm at room temperature. When temperature increases to about 120° C., the pump LED emits radiation about 405 nm. Typically, high current and/or high temperature are the main causes of wavelength shift. For example, for each increase of 23° C. in operating temperature, the wavelength of the radiation emitted by the pump LED increases by 1 nm. The encapsulant and the phosphor material used in various embodiments of the invention can compensate for the wavelength shift.

FIG. 5 illustrates carrier spreading in multi-quantum well (MQW) LEDs emitting at 450 nm (left) and 420 nm (right). In a 450 nm-emitting scheme, the energy barriers are larger, which can impede spreading of the holes between the quantum wells. As shown, electrons are more or less spread evenly, while holes are not. In contrast, in a 420 nm-emitting scheme, the energy barriers are lower and hole spreading is thus improved, thereby increasing the effective volume of the active region. Better optimized carrier spreading is achieved by embodiments of the present invention. More specifically, carrier confinement can be reduced compared to that a 450 nm pump LED, which enables better carrier spreading in a MQW system. Therefore, one can employ a thick active region (for instance, more than 10 nm thick or more than 50 nm thick) and inject carriers efficiently across this active region, as illustrated in FIG. 5.

FIG. 6 illustrates an embodiment of the invention where five pump LEDs are arranged in an array. In this implementation the LEDs emit at about 405 nm to 430 nm. The LEDs are disposed a phosphor mix that is specifically made for color conversion. As described above, the phosphor mix comprises phosphor materials which have a high absorption of the light emitted by the pump LEDs. For example, the phosphor mix comprises one or more of the following materials: BaMgAl₁₀O17:Eu²⁺, Sr₁₀(PO₄)₆Cl₂:E, LaAl(Si_(6-z)Al_(z))N_(10-z)O_(z):Ce³⁺, a-Sialon:Ce³⁺, (Y, La)—Si—O—N:Ce³⁺, Gd_(1-x)Sr_(2+x)AlO_(5-x)F_(x):Ce³⁺. The phosphor mix is prepared for converting the light from the pump LEDs to light in other colors, such as red, green, and/or blue. When the light in different colors is combined, preferably substantially white light is produced. The mix of phosphor materials is disposed in an encapsulant, which is substantially transparent to both pump-LED and phosphor-emitted light. In a preferred embodiment, the encapsulant is specifically configured to improve light extraction efficiency and is formed from polymeric species.

FIG. 7 illustrates an embodiment of the invention where the pump LEDs are arranged in an array, and the phosphor composition is varied spatially in a pixilated configuration. Here, separate spatial regions perform conversion to red, green, and blue light. As shown in FIG. 7, five LEDs are arranged in an array configured to pump phosphor materials. Single-color phosphor materials in different colors absorb radiation emitted by the LEDs and re-emit light in a color associated with the phosphor material. The phosphor materials are arranged in a pixelated fashion over the LEDs. The pixelated pattern is specifically created to create a mix of emissions whose combination produces light that is substantially white in color.

In a preferred embodiment, the LEDs emit radiation in substantially the same color (e.g., about 405 nm to 430 nm in wavelength), and the radiation from the LEDs pumps the single-color phosphor materials that are in different spatial locations. In return, the colored phosphor materials emit colored light. For example, the phosphor materials, as shown in FIG. 7, emit respectively red, green, and blue light. In the configuration shown in FIG. 7, the type of pump LEDs and/or the phosphors can be varied across the array based on the color needed and the type of LEDs used.

FIG. 8 illustrates an embodiment of the invention where the pump LEDs are arranged in an array, and two LED emission wavelengths are employed. Short-wavelength LEDs (about 405 nm to 430 nm) pump red and green phosphors, while longer-wavelength LEDs (440 nm to 460 nm) emit blue light. As shown in FIG. 8, five LED devices form an LED array positioned on a substrate or submount. More specifically, the LED device in the middle emits blue light (e.g., wavelength of about 440 nm to 460 nm), while the other LED devices emit radiation in substantially violet (e.g., about 405 nm to 430 nm) wavelength range. The violet LED devices are disposed in colored phosphor materials, where the LED devices pump the phosphor material which emits a colored light, e.g. red light. Similarly, the green phosphor material, upon absorbing substantially violet radiation, emits green light. The blue LED device is not disposed within a phosphor material, and as a result the blue light generated by the blue LED devices is emitted directly.

Depending on the application, the LEDs can be arranged in array geometry using several pump LEDs in combination with blue or red LEDs that are not configured to pump phosphor material. It is to be appreciated that the arrangement of LEDs as shown in FIG. 8 can help reduce light absorption. For example, inter-die absorption is reduced at about 405 nm to 430 nm relative to 400 nm (because of the lower substrate absorption), which is an additional advantage of using a longer wavelength pump LED.

Different arrangements of phosphor materials and LED devices enable different light colors to be obtained. In a preferred embodiment, the LED devices are grown on a non-polar or semi-polar substrate. In certain embodiments, the LED devices can be grown on a low-dislocation-density substrate (<1×10⁷ dislocations/cm²) to enable reliable operation at high current density and high temperature.

Wavelength conversion materials can be ceramic or semiconductor particle phosphors, ceramic or semiconductor plate phosphors, organic or inorganic down converters, up converters (anti-stokes), nano-particles and other materials which provide wavelength conversion. Some examples are listed below

-   (Sr_(n),Ca_(1-n))₁₀(PO₄)₆*B₂O₃:Eu²⁺(wherein 0≤n≤1) -   (Ba, Sr, Ca)₅(PO₄)₃(Cl, F, Br, OH):Eu²⁺, Mn²⁺ -   (Ba, Sr, Ca)BPO₅:Eu²⁺, Mn²⁺ -   Sr₂Si₃O₈*2SrCl₂:Eu²⁺ -   (Ca, Sr, Ba)₃MgSi₂O₈:Eu²⁺, Mn²⁺ -   BaAl₈O₁₃:Eu²⁺ -   2SrO*0.84P₂O₅*0.16B₂O₃:Eu²⁺ -   (Ba, Sr, Ca)MgAl₁₀O₁₇:Eu²⁺, Mn²⁺ -   K₂SiF₆:Mn⁴⁺ -   (Ba, Sr, Ca)Al₂O₄:Eu²⁺ -   (Y, Gd, Lu, Sc, La)BO₃:Ce³⁺, Tb³⁺ -   (Ba, Sr, Ca)₂(Mg, Zn)Si₂O₇:Eu²⁺ -   (Mg, Ca, Sr, Ba, Zn)₂Si_(1-x)O_(4-2x):Eu²⁺(wherein 0≤x≤0.2) -   (Sr, Ca, Ba)(Al, Ga)₂S₄:Eu²⁺ -   (Ca, Sr)₈(Mg, Zn)(SiO₄)₄Cl₂:Eu²⁺, Mn²⁺ -   Na₂Gd₂B₂O₇:Ce³⁺, Tb³⁺ -   (Sr, Ca, Ba, Mg, Zn)₂P₂O₇:Eu²⁺, Mn²⁺ -   (Gd, Y, Lu, La)₂O₃:Eu³⁺, Bi³⁺ -   (Gd, Y, Lu, La)₂O₂S:Eu³⁺, Bi³⁺ -   (Gd, Y, Lu, La)VO₄:Eu³⁺, Bi³⁺ -   (Ca, Sr)S:Eu²⁺, Ce³⁺ -   (Y, Gd, Tb, La, Sm, Pr, Lu)₃(Sc, Al, Ga)_(5-n)O_(12-3/2n):Ce³⁺     (wherein 0≤n≤0.5) -   ZnS:Cu+, Cl− -   (Y, Lu, Th)₃Al₅O₁₂:Ce³⁺ -   ZnS:Cu+, Al3+ -   ZnS:Ag+, Al3+ -   ZnS:Ag+,Cl− -   (Ca, Sr) Ga₂S₄: Eu²⁺ -   SrY₂S₄:Eu²⁺ -   CaLa₂S₄:Ce³⁺ -   (Ba, Sr, Ca)MgP₂O₇:Eu²⁺, Mn²⁺ -   (Y, Lu)₂WO₆:Eu³⁺, Mo⁶⁺ -   CaWO₄ -   (Y, Gd, La)₂O₂S:Eu³⁺ -   (Y, Gd, La)₂O₃:Eu³⁺ -   (Ba, Sr, Ca)_(n)Si_(n)N_(n):Eu²⁺ (where 2n+4=3n) -   Ca₃(SiO₄)Cl₂:Eu²⁺ -   (Y, Lu, Gd)_(2-n)Ca_(n)Si₄N_(6+n)C_(1-n):Ce³⁺, (wherein 0≤n≤0.5) -   (Lu, Ca, Li, Mg, Y) alpha-SiAlON doped with Eu²⁺ and/or Ce³⁺ -   (Ca, Sr, Ba)SiO₂N₂:Eu²⁺, Ce³⁺ -   (Sr, Ca)AlSiN₃:Eu²⁺ -   CaAlSi(ON)₃:Eu²⁺ -   Sr₁₀(PO₄)₆Cl₂:Eu²⁺ -   (BaSi)O₁₂N₂:Eu²⁺ -   SrSi₂(O, Cl)₂N₂:Eu²⁺ -   (Ba, Sr)Si₂(O, Cl)₂N₂:Eu²⁺ -   LiM₂O₈:Eu³⁺ where M=(W or Mo)

In the list above, it is understood that when a phosphor has two or more dopant ions (i.e. those ions following the colon in the above phosphors), this means that the phosphor has at least one (but not necessarily all) of those dopant ions within the material. That is, as understood by those skilled in the art, this type of notation means that the phosphor can include any or all of those specified ions as dopants in the formulation.

In certain embodiments, quantum-dot-based phosphors are used for color conversion purposes. Quantum dot materials are a family of semiconductor and rare earth doped oxide nanocrystals whose size and chemistry determine their luminescent characteristics. Typical chemistries for the semiconductor quantum dots include well known (ZnxCd1-x) Se [x=0 . . . 1], (Znx, Cd1-x)Se[x=0 . . . 1], Al(AsxP_(1-x)) [x=0 . . . 1], (Znx, Cd1-x)Te[x=0 . . . 1], Ti(AsxP1-x) [x=0 . . . 1], In(AsxP1-x) [x=0 . . . 1], (AlxGa1-x)Sb [x=0 . . . 1], (Hgx, Cd1-x)Te[x=0 . . . 1] zincblende semiconductor crystal structures. Published examples of rare-earth doped oxide nanocrystals include Y2O3 :Sm3+, (Y, Gd)2O3 :Eu3+, Y2O3 :Bi, Y2O3 :Tb, Gd2SiO5 :Ce, Y2SiO5 :Ce, Lu2SiO5:Ce, Y3Al5)12:Ce but should not exclude other simple oxides or orthosilicates.

In certain embodiments, the present invention provides a two-phosphor violet pumped white LED, which can be used for illumination. In a specific embodiment, a cyan phosphor and an orange phosphor are used with violet-pumped LED chips. For example, violet chips emitting radiation at wavelength of about 400-440 nm are provided. In this configuration, the emission of the two phosphors substantially determine the chromaticity and color quality of the white light. For example, color is determined primarily by the mix of two phosphors, and pump light leakage is typically minimal and not very eye-sensitive. This approach combines the advantages of reduced phosphor complexity and cost with the simplified color tuning of a two-phosphor system. Moreover, the low eye sensitivity of the violet pump ensures that color targeting in manufacturing with high yield is provided, unlike the case for blue-pumped systems in which case tight control over blue light leakage is necessary for maintaining tight control over chromaticity spread in high-volume manufacturing.

FIG. 9 is a diagram illustrating the emission spectrum of a two-phosphor violet pumped white LED. In this example, a cyan phosphor G1758™ and an orange phosphor O6040™ from Intematix Corporation are combined and pumped by a ˜420 nm emitting LED. It is to be appreciated that other types of cyan and orange phosphor can be used as well. Other phosphor combinations and other choices of pump emission wavelength in such a configuration are possible. In FIG. 9, the violet pump LED leakage is varied +/−20% relative to an absolute level of 5% leakage. For all three cases, the color rendering index (CRI) is at about 83 at a CCT of approximately 2700K. The variation in chromaticity between the three spectra is very small, with the total deviation from the Planckian at less than 0.004 in terms of u′, v′. The lumen efficacy is at about 301-305 lm/Wopt. One of the benefits is that the configuration uses long wavelength pump chips (e.g., in this case 420 nm), which increases light extraction efficiency and package efficiency, reduces forward voltage and Stokes loss, compared to shorter wavelength pump LEDs. In addition, by avoiding blue phosphor, this configuration removes one component in loss in efficiency, reduces overall phosphor loading, and reduces cost and complexity.

While the above is a full description of the specific embodiments, various modifications, alternative constructions, and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims. 

The invention claimed is:
 1. A light emitting system for emitting white light, said light emitting system comprising: a first set of LEDs comprising at least one first LED and emitting a first light having a first peak wavelength in a range 405-430 nm, a second set of LEDs comprising at least one second LED and emitting a second light having a second peak wavelength which is longer than the first peak wavelength; and a wavelength-converting material, configured to substantially absorb a portion of light from one of the first or second sets of LEDs and convert it to a third light, without substantially absorbing light from the other set of LEDs; wherein said white light comprise at least a portion of said first, second and third lights.
 2. The system of claim 1, wherein the second set of LEDs are blue LEDs.
 3. The system of claim 1, wherein of the light from one of the first or second sets of LEDs is the first light.
 4. The system of claim 1, wherein the wavelength-converting material is disposed over one of the first or second sets of LEDs, and is not disposed over the other set of LED.
 5. The system of claim 4, wherein the one of the first or second sets of LEDs is the first set of LEDs.
 6. The system of claim 1, wherein the wavelength-converting material comprises a plurality of wavelength-converting materials.
 7. The system of claim 1, wherein the wavelength-converting material comprises a green phosphor and a red phosphor.
 8. The system of claim 1, wherein the wavelength-converting material does not include a blue phosphor.
 9. The system of claim 1, wherein the first set of LEDs is characterized by an internal quantum efficiency of at least 70% when operated at a current density of 100 A·cm-2.
 10. The system of claim 1, wherein the second set of LEDs are blue LEDs, and the wavelength converting material is disposed over the first set of LEDs and is not disposed over the second set of LEDs.
 11. A method of operating a light emitting system, said method comprising: receiving power at one or more first LEDs at a current density of at least 100 A·cm-2 to emit a first light having a first peak wavelength in a range 405-430 nm with an internal quantum efficiency of at least 70%; receiving power at one or more one second LEDs to emit a second light having a second peak wavelength which is longer than the first peak wavelength; and absorbing a portion of light from one of the first or second LEDs at the wavelength-converting material, and converting said portion to a third light, without substantially absorbing light from the other set of LEDs, such that the system emits a substantially white light comprising a combination of at least a portion of the first, second and third lights.
 12. The method of claim 1, wherein the second LEDs are blue LEDs.
 13. The method of claim 1, wherein of the light from one of the first or second LEDs is the first light.
 14. The method of claim 1, wherein the wavelength-converting material is disposed over one of the first or second LEDs, and is not disposed over the other LED.
 15. The method of claim 4, wherein the one of the first or second LEDs is the first LEDs.
 16. The method of claim 1, wherein the wavelength-converting material comprises a plurality of wavelength-converting materials.
 17. The method of claim 1, wherein the wavelength-converting material comprises a green phosphor and a red phosphor.
 18. The method of claim 1, wherein the wavelength-converting material does not include a blue phosphor.
 19. The method of claim 1, wherein the first LEDs is characterized by an internal quantum efficiency of at least 70% when operated at a current density of 100 A·cm-2.
 20. The method of claim 1, wherein the second LEDs are blue LEDs, and the wavelength converting material is disposed over the first LEDs and is not disposed over the second LEDs. 